The present disclosure relates to a scanning optical device configured to scan and expose a photosensitive drum with a laser beam and an image forming apparatus provided with the scanning optical unit.
An image forming apparatus is generally provided with a scanning optical device constituted of a laser scanning unit exposing a photosensitive drum with a laser beam and forming an electrostatic latent image on the photosensitive drum. The scanning optical device includes a light source emitting the laser beam, a deflector deflecting the laser beam, optical component groups containing lenses and mirrors and an optical box storing the light source, the deflector and the optical groups.
The scanning optical device has an opposed scanning type configured such that a pair of optical component groups are symmetrically arranged with respect to the deflector in the optical box and radiate laser beams on two photosensitive drums to form an latent image on each photoconductive drum. In such the scanning optical device of an opposed scanning type, a fθ lens focusing the laser beam in a sub-scanning direction may be deformed so as to curve in the sub-scanning direction due to temperature rising in the optical box. When the fθ lens is deformed in the sub-scanning direction in the scanning optical device of an opposed scanning type, the images formed on two photoconductive drums curve in opposite directions each other in the sub-scanning direction. As a result, color registration cannot be aligned and, therefore, deterioration of image quality, such as color irregularity and the others, is caused.
The lenses, such as the fθ lens, are often adhered to the optical box using an adhesive in view of material cost and automation of assembly. In such a case, because of a difference in linear expansion coefficients between a material of the lens and a material of the optical box, it is necessary to absorb a difference between the deformed amounts of the lens and the optical box when they are thermally deformed. Usually, since a linier expansion coefficient of the optical box is smaller than that of the lens, a deformed amount of the lens is larger than that of the optical box. Therefore, if the lens may be adhered to the optical box with multiple adhesion positions, the difference between the deformed amounts cannot be absorbed to cause a partial distorsion on the lens and peeling of the adhesive. On the other hand, if the lens may be adhered at one adhesion position near the center, although the difference between the deformed amounts can be absorbed, some problems may be occurred. For example, a posture of the lens may become unstable depending on the adhesion position of the lens, the lens may be easily detached due to lack of adhesive strength and vibration may occur easily.
Therefore, in the scanning optical device, in order to support the lenses stably while absorbing the difference between the deformed amounts of the lens and the optical box, the both ends of the fθ lens may be pressed with plate springs. Alternatively, one end of the fθ lens may be pressed with a plate spring while pressing the other end against a positioning rib.
However, in the above-mentioned scanning optical device employing the plate spring, although the difference in the linear expansion coefficients can be absorbed by deformation of the plate spring, if impact and vibration may be applied during transportation or another situation, some problems, such as a displacement of the fθ lens from a predetermined position and difficulty in automation of assembly, may be caused.